Thiazolium-Functionalized Poly(terphenyl) Anion Exchange Membranes with Improved Dimensional Stability for Alkaline Water Electrolysis

Abstract Sulfur-containing ionomers have emerged as promising materials for alkaline water electrolysis. In this work, a series of anion exchange membranes bearing different cyclic cationic groups were rationally designed and synthesized. A rigid poly(terphenyl) backbone free of aryl ether linkages was synthesized via superacid-catalyzed polymerization. Thiazole, imidazole, and piperidine side groups were introduced and subsequently quaternized to yield the corresponding thiazolium (QPTTz), imidazolium (QPTIm), and piperidinium (QPTP) membranes. All membranes exhibited comparable ion exchange capacities ranging from 2.64 to 2.75 mmol g–1. Under identical test conditions, QPTTz showed markedly reduced water uptake (19.31%) and swelling ratio (8.51%) at 80 °C while delivering a high hydroxide conductivity of 102.40 mS cm–1, close to that of QPTIm (105.62 mS cm–1), and a low apparent activation energy of 15.54 kJ mol–1. Atomic force microscopy phase images suggest that QPTTz possesses a finer and more homogeneous microphase-separated morphology. Following alkaline stability testing, QPTTz retains 91.16% of its original OH– conductivity, significantly outperforming its imidazolium and piperidinium counterparts. Theoretical calculations further provide insight into the enhanced stability of the thiazolium cation toward chemical degradation under alkaline conditions. When assembled into an AEM water electrolyzer, the QPTTz membrane delivered lower and more stable cell voltages (1.83–1.89 V at 0.5 A cm–2) during continuous operation. These results identify thiazolium functionalization as an effective strategy for low-swelling, durable AEMs for sustainable hydrogen production.